Maggie Jones described one study in an eye-opening article in the New York Times magazine in which subjects were allowed to sleep four, six or eight hours a night for two weeks and were then tested with the psychomotor vigilance task, or P.V.T., which measures sustained attention, a gold standard of sleepiness measures:

Not surprisingly, those who had eight hours of sleep hardly had any attention lapses and no cognitive declines over the 14 days of the study. What was interesting was that those in the four- and six-hour groups had P.V.T. results that declined steadily with almost each passing day. Though the four-hour subjects performed far worse, the six-hour group also consistently fell off-task. By the sixth day, 25 percent of the six-hour group was falling asleep at the computer. And at the end of the study, they were lapsing fives times as much as they did the first day.

So six hours, which many of us get, is not cutting the mustard. But then the author goes on to ask about those who sleep not six, not eight, but seven hours a night, noting that the average American gets 6.9 hours of sleep.

Why 7 Hours Isn't Enough

She reported that in another study of seven-hour sleepers, "their response time on the P.V.T. slowed and continued to do so for three days, before stabilizing at lower levels than when they started." So sleeping seven, which most of us think is ok, is not either.
But people are different and some of us can get away with one or two nights of less sleep. If you're lucky, you might not feel the effects of a five hour sleep the first day--some people can go a few days before their performance starts to go down hill.

Chronically Getting Less Hurts Performance
The problem is, for many of us, sleeping less is chronic, not just an once a week occurrence. And you can be sure of one thing. You probably won't realize when you're flagging. The subjects in the studies reported they were slightly sleepy, but, wrote Jones,

they insisted they had adjusted to their new state. Even 14 days into the study, they said sleepiness was not affecting them. In fact, their performance had tanked. In other words, the sleep-deprived among us are lousy judges of our own sleep needs. We are not nearly as sharp as we think we are.

So maybe it's time to rethink that less is more macho attitude that is prevalent in some business circles and get more sleep. Eight hours is ideal for 95 percent of us--any less and our performance will suffer.

Laurie Tarkanis an award-winning health journalist who writes for the New York Times, national magazines and websites including Health, Prevention, Ladies Home Journal, iVillage and the Huffington Post. Follow her on twitter.

Laurie Tarkan is an award-winning health journalist who writes for The New York Times and many national magazines. She is a contributing editor at Fit Pregnancy magazine and the author of three books, Perfect Hormone Balance for Fertility, Perfect Hormone Balance for Pregnancy and My Mother's Breast: Daughters Ace Their Mothers' Cancer.. You can follow her on Twitter at @LaurieTarkan.